1. Field of the Invention
The present invention relates to an organic EL element, and also to a method of forming it.
2. Description of the Related Art
An organic EL (Electroluminescence) element has a basic structure in which a lower electrode is formed on a substrate, an organic material layer including a luminescent layer is formed in a single- or multi-layer form on the lower electrode, and an upper electrode is formed on the organic material layer, whereby the organic material layer is interposed between the pair of electrodes. One of the upper and lower electrodes is used as an anode, and the other electrode as a cathode. When a voltage is applied between the electrodes, electrons which are injected and transported from the cathode into the organic material layer recombine with holes which are injected and transported from the anode, thereby causing luminescence. Recently, such an organic EL element receives attention as a thin luminescence element which can provide surface luminescence, and which constitutes a light source for various purposes or a display unit of a self-luminous thin display device.
JP-A-2003-45676 proposes an organic EL element which is an improvement of an organic EL element having the above-described basic structure, and in which bright luminescence can be obtained and the life can be prolonged. FIG. 1 is a diagram illustrating the conventional art. In the organic EL element of the conventional art, a plurality of light-emitting units (3-1, 3-2, . . . , 3-n) are disposed on a substrate 1 and between an anode 2 and a cathode 5 which are opposed to each other, and the light-emitting units are separated from one another by layers (4-1, 4-2, . . . , 4-(n-1)) respectively forming a single equipotential plane.
In the organic EL element, each of the layers (4-1, 4-2, . . . , 4-(n-1)) which separate the light-emitting units (3-1, 3-2, . . . , 3-n) from one another has a function of generating holes for the light-emitting unit which is adjacent on the side of the cathode, and electrons for that which is adjacent on the side of the anode. The above-described basic structure is formed for each of the light-emitting units. The resulting structures are connected in series.
In the organic EL element, the light-emitting units exhibiting organic EL luminescence are connected in series, and the driving voltage is equal to the total sum of potentials consumed by the light-emitting units. A plurality of luminescences produced respectively in the light-emitting units can be combined, and the resulting composite light can be emitted. Therefore, high luminance can be obtained in accordance with the number of the light-emitting units.
Usually, the luminance of an organic EL element is substantially proportional to the current density. In order to obtain high luminance, therefore, a high current density is inevitably required. When the current density is set to be high, there arises a disadvantage that the life of the element is shortened. By contrast, in the organic EL element disclosed in JP-A-2003-45676, high luminance can be obtained by increasing the number of light-emitting units, and, even when the unit number is increased, the current density in each of the light-emitting units is not changed. Therefore, it is possible to realize high luminance without sacrificing the life of the element.
The method in which plural light-emitting units are connected in series via conductive layers having a function of generating charges, and luminescences produced respectively in the light-emitting units are combined together to obtain composite light is reported also in Dai 49 kai oyobutsurigaku kanren rengo koenkai yokoshu p. 1308 (27p-YL-3) in addition to JP-A-2003-45676. A technique in which plural light-emitting units produces luminescences of different colors, and the luminescences are superimposed to obtain a synthetic color is disclosed in Dai 63 kai oyobutsurigakkai gakujutsu koenkai yokoshu p. 1165 (27a-ZL-12).
As described above, in an organic EL element in which plural light-emitting units exhibiting organic EL luminescence are connected in series, the same current flows through the series-connected light-emitting units, and hence the current density cannot be adjusted for each of the light-emitting units. Therefore, the chromaticity of the mixed color output from the whole organic EL element is automatically determined by the luminance efficiency (current-luminance efficiency) of each of the light-emitting units with respect to the density of the common current flowing through all of the light-emitting units.
In other words, in such an organic EL element, even when the luminous color which is obtained as a mixed color of the light-emitting units is to be set to a desired chromaticity, it is impossible to adjust the luminance of each color for each of the light-emitting units, and hence the chromaticity of the mixed color is determined by the current-luminance efficiency of a selected luminescent material, thereby causing a problem in that the obtained mixed color fails to have the desired chromaticity.
A case where such an organic EL element is used as a white light source due to mixture of three colors of R (Red), G (Green), and B (Blue) will be considered. Organic EL luminescent materials have different current-luminance efficiencies for the colors of R (Red), G (Green), and B (Blue), and the luminance mixing ratio of RGB (R:G:B=3:6:1) for obtaining white by color mixture cannot be obtained only by selecting a luminescent material. Therefore, there arises a problem in that white cannot be obtained by color mixture of light-emitting units.
Luminescent materials which are useful in light-emitting units include those which exhibit luminescence (fluorescence) when the state is returned from the singlet excited state to the ground state, and those which exhibit luminescence (phosphorescence) when the state is returned from the triplet excited state to the ground state. Usually, the quantum efficiency of fluorescence is ⅓ to ¼ of that of phosphorescence. Consequently, there is a problem in that, when color mixture of RGB is obtained by using both kinds of luminescences, the obtained color is hued white.